1. Field of the Invention
This invention relates to circuit breakers and more particularly to an electronic circuit breaker which responds to sputtering arc faults as well as overcurrent faults and which can be combined with a ground fault detector. The invention further relates to the shared use of a di/dt current sensor by the line-to-neutral fault detector and the ground fault detector.
2. Background Information
Conventional residential circuit breakers have a thermal trip device which responds to persistent overcurrents of moderate magnitude to provide a delayed trip, and a magnetic trip device which responds instantaneously to overcurrents of large magnitude. Thus, the fault current must reach a predetermined magnitude, for example ten times rated current for the instantaneous trip to occur, or the overcurrent must sustain a predetermined average value over a given time interval to implement the delayed trip. There is a type of fault, however, which may not produce either the peak magnitude required for the instantaneous magnetic trip or the sustained average overcurrent necessary for the delayed trip, yet may pose a fire hazard. This is the intermittent or sputtering arc-type of fault. Such a fault can occur, for instance, between two conductors that are in close proximity, but not touching so that an arc is struck between the conductors. This arc can produce a temperature high enough to melt the copper in the conductor. The melted droplets of copper can ignite flammable material in the vicinity. The resistance of the wire may be high enough to limit the peak current and the ac current cyclically passes through zero to extinguish the arc so that the average current is low. Thus, the conventional circuit breaker does not respond to the fault, although a hazard exists. This is especially true in the case of a stranded wire extension cord where an individual strand can be melted at a relatively low fault current.
As sufficient voltage is required to strike the arc of a sputtering arc fault, this type of fault typically occurs near the peak of the ac voltage waveform thereby resulting in a step increase in current. Switching of some residential loads also produces step increases in current. For instance, an iron which is turned on at the peak of the voltage waveform results in a step increase in current; however, the magnitude of the step is less than the rated current of the circuit breaker. In addition, inrush currents, such as those produced by starting of a motor, have a high initial value, but do not have a high rate of change of current, di/dt, as this is limited by the motor inductance.
Ground fault protection circuits provided in a separate receptacle or as part of a circuit breaker are known. One type of ground fault detector, used with a grounded neutral electrical system and known as the dormant oscillator detector, includes one sense coil on both the line and neutral conductors which detects line-to-ground faults and a second sense coil on the neutral conductor alone which senses neutral to ground faults by generating an oscillation when coupled to the first coil through the neutral-to-ground fault.
There remains a need for a circuit breaker that can distinguish between sputtering arc type faults and inrush currents.
There is also a need for such a circuit breaker which is as compact as possible.
There is an additional need for such a circuit breaker which can provide separate sensitivities to overcurrent line-to-neutral faults and sputtering arc type faults.
There is also a need for providing line-to-neutral and ground fault protection in a compact economical unit.